As electronics have become increasingly portable and sophisticated, consumers have come to expect quality power sources for these products. Typically the best solution is to use a rechargeable battery or battery pack. Such battery packs must be very robust in order to provide as many charge/discharge cycles as possible. In addition, since battery packs are energy storage devices, they must be safe. This is especially true of some of the newer varieties of batteries that have recently become available, for example, lithium ion batteries which have a significantly higher energy density compared to established battery systems such as nickel-cadmium. In order to achieve both safety and reliability, it is necessary for designers of such battery packs to assume a certain level of mistreatment, either intentionally or otherwise, once in the hands of the end user.
One event that decreases reliability, and has occurred with increasing frequency is accidental deep discharge of the battery pack. This occurs when a device is left on for an extended period of time, such as when the user forgets about it, and the battery is discharged far beyond its normal discharge level. For newer systems such as nickel metal hydride, lithium ion, and lithium polymer, this could cause a battery failure. At best, the cycle life of the battery is significantly shortened; at an extreme, it can create a safety risk when recharged.
Another potential problem unique to newer battery systems, particularly lithium ion and lithium polymer, is overcharge. It is well known that lithium metal is highly reactive. In lithium ion cells, there is initially no lithium metal available in the cell. So long as the battery voltage does not exceed a threshold voltage, none will form. If the battery is charged at a voltage above this threshold level, then the battery cells experience what is referred to as an over-voltage condition. This over-voltage condition causes lithium metal to precipitate out of solution in a reaction similar to an electrochemical plating process. Over time, a safety risk could develop as more lithium metal builds up. Lithium polymer cells likewise experience a similar phenomena upon overcharge.
In a typical system, the charger is designed to avoid over-voltage conditions. A variety of failure modes can, however, cause a charger to continue charging once the threshold voltage is reached. This type of failure is commonly referred to as a runaway charger. In order to protect against such a failure, some sort of battery disconnect switch is needed to keep the battery cells from experiencing any over-voltage charging. Ideally such a switch, once activated should still allow the battery pack to be subsequently discharged.
Battery disconnect switches have been advantageously employed in the past to address both under-voltage and over-voltage conditions. However, their cost and complexity often offset any advantage a newer battery system may offer, which is typically already more expensive than conventional systems. For example, there are battery pack resident low voltage, or more commonly, under-voltage, switch circuits in use for lithium ion batteries. However, these circuits, once activated, require a specific signal from a charger to "wake up" the battery switch circuit and allow use of the battery. This requires a certain level of complexity on the part of the charger. Similarly, battery pack resident over-voltage disconnect circuits have been used in a number of lithium ion battery packs. However, they typically use multiple comparators, precision voltage references, and in many cases, custom made integrated circuits.
Accordingly, there is a need for a simple means by which a battery can be disconnected from a load when the battery has been normally discharged, i.e., prior to any deep discharge, and still be rechargeable. Likewise, there is a need for a simple means to disconnect a battery before an overcharge condition occurs so as to avoid any safety risk. Ideally, such requirements can be met by a single means.